In recent years, research and development concerning the so-called high density optical disc have been proceeding. In the high density optical disc, the recording density of an optical information recording medium (an optical disc) is increased by the use of a blue laser beam having a wavelength of about 400 nm, and then the storage capacity of the optical information recording medium is increased.
As the standards of the high density optical disc, for example, a standard regulating the numerical aperture (NA) on an image side of an objective lens to be about 0.85 and the thickness of a protective substrate to be about 0.1 mm, and a standard regulating the NA and the thickness of the protective substrate to be about 0.65 and about 0.6 mm or less, respectively, at the same level as those of a conventional digital versatile disc (DVD) are known. In the following description, a high density optical disc having an NA of about 0.65 and the thickness of a protective substrate of about 0.6 mm will be referred to as an “advanced optical disc (AOD)”.
Then, various techniques pertaining to optical pickup devices compatible with such high density optical discs and conventionally wide used optical discs such as DVD's and compact discs (CD's) have been proposed. For example, one of them is disclosed in Unexamined Japanese Patent Application Publication No. 2002-298422.
Because the compatible optical pickup device uses optical beams each having a different wavelength from each other (for example, about 400 nm for the AOD, and about 650 nm for the DVD), the compatible optical pickup device is required to correct spherical chromatic aberrations caused by the wavelength difference into a level at which no troubles are caused practically.
Moreover, for example, when the so-called mode hop, at which the wavelength of a light beam instantaneously changes, is generated at the time of increase of the power of the light beam emitted from a light source, the problem that the position of a converging spot formed on an optical axis shifts from an information recording surface of an optical disc into an optical axis direction is caused. Consequently, it is necessary to perform a correction for decreasing the quantity of a change of the converging spot in the optical axis direction before and after the wavelength change to suppress axial chromatic aberrations.
In particular, in the AOD, the quantity of the wavelength change at the time of the mode hop is large owing to the relatively large NA of about 0.65 and the short of the wavelength of the light beam being about 400 nm. Consequently, there is the problem that the quantity of the shift of the converging spot is large.
The objective lens disclosed in the above-mentioned publication suppresses a chromatic aberration on an axis caused by a change of the wavelength of a blue laser beam without excessively correcting the chromatic aberration on the axis in a DVD, by proving a diffractive structure (a diffraction lens structure) on an optical surface on one side of an objective lens, and by adopting beams having different orders of diffraction on a short wavelength side and a long wavelength side.
However, because the publication does not disclose any techniques for correcting the spherical chromatic aberration caused by a difference between wavelengths, there is the case where information cannot be recorded or reproduced appropriately when a plurality of recording media such as a high density optical disc and a DVD is used.